1. Field of the Invention
The present invention relates to an acceleration detector which adopts an unique arrangement of resistance elements having a piezo resistance effect for improving the sensitivity thereof.
2. Description of the Prior Art
As an acceleration detector for detecting acceleration applied thereto, diaphragm- and beam-type acceleration detectors are known. For example, as shown in FIGS. 21 and 22, a diaphragm-type acceleration detector 1C comprises a frame 10C defining an open space therein and having a top face 11C and bottom face 12C, a bottom cover 20C, a thin-sheet resilient member 30C, and a weight 40C. The resilient member 30C is jointed at its periphery to the top face 11C of the frame 10C. The weight 40C is hung down from a center of the resilient member 30C through a neck portion 41C. The neck portion 41C is fixed with a center portion 31C of the resilient member 30C in such a manner as to cause an elastic deformation of the resilient member when the weight 40C is displaced with respect to the frame 10C by receiving an acceleration. These parts of the acceleration detector 1C are made of a semi-conductor material. Resistance elements R having a piezo resistance effect are formed in the resilient member 30C to detect three components of acceleration applied to the detector 1C with respect to X-, Y- and Z- axes of an orthogonal coordinate system in such a manner that electric resistance of the resistance elements R is varied in response to strain of the resistance elements accompanied with the elastic deformation of the resilient member. The acceleration is determined in accordance with a variation of the electric resistance by an acceleration determining section. The bottom cover 20C is useful as a stopper for preventing a breakage of the resilient member 30C when receiving an excess acceleration. Numeral "62" designates a bonding pad for wiring a conductor pattern.
U.S. Pat. No. 4,967,605 discloses an arrangement of resistance elements R for a diaphragm-type acceleration detector, as shown in FIG. 24. The resistance elements R consisting of a first set of four resistance elements RX1-RX4 for detecting a first component of acceleration with respect to the X-axis, a second set of four resistance elements RY1-RY4 for detecting a second component of acceleration with respect to the Y-axis, and a third set of four resistance elements RZ1-RZ4 for detecting a third component of acceleration with respect to the Z-axis. When X- and Y-axes are set on a plane of a thin-sheet resilient member of the detector, the resistance elements RX1-RX4 of the first set are aligned on the X-axis, and assembled as a first bridge circuit, as shown in FIG. 25A. The resistance elements RY1-RY4 of the second set are aligned on the Y-axis, and assembled as a second bridge circuit, as shown in FIG. 25B. In addition, the resistance elements RZ1-RZ4 of the third set are aligned parallel to the X-axis, and assembled as a third bridge circuit as shown in FIG. 25C. When a predetermined voltage or current is delivered from a power source to each bridge circuit, the respective bridge voltages are measured by voltage meters Vx, Vy and Vz. Therefore, the three components of acceleration can be detected independently with respect to the X-, Y- and Z-axes.
On the other hand, a beam-type acceleration detector 1D is the substantially same structure as the diaphragm-type acceleration detector except that a resilient member 30D is formed with four rectangular holes 32D around a center portion 31D thereof so as to be shaped into a cross beam configuration, as shown in FIG. 23. In case that the beam-type acceleration detector 1D adopts the same arrangement of the resistance elements of FIG. 24, stresses received the resistance elements when acceleration is applied to a weight 40D of the detector are analyzed as below. That is, when no acceleration is applied to the detector, stress is not applied to the resistance elements, as shown in FIG. 26A. For example, when acceleration F1 is applied to the detector upwardly in the Z-axis direction, as shown in FIG. 26B, each of the resistant elements RX1, RX4, RY1, RY4, RZ1, and RZ4 receives a compressive stress .sigma.1 which is indicated by minus sign in FIG. 26C. Each of the resistance elements RX2, RX3, RY2, RY3, RZ2 and RZ3 receives a tensile stress .sigma.2 which is indicated by plus sign in FIG. 26C. The tensile stress .sigma.2 is equal to the absolute value of the compressive stress .sigma.1. Therefore, when acceleration is applied to the detector in the Z axis direction, a sensitivity of the acceleration detector is not dominated by the arrangement of the resistance elements.
However, when acceleration F2 is applied to the detector in the X-axis direction, as shown in FIG. 26D, each of the resistant elements RX1 and RZ1 receives a tensile stress .sigma.3, and each of the resistance elements RX3 and RZ3 receives a tensile stress .sigma.4 which is twice as large as the stress .sigma.3. On the contrary, each of the resistance elements RX4 and RZ4 receives a compressive stress .sigma.6, and each of the resistance elements RX3 and RZ3 receives a compressive stress .sigma.5 which is twice as large as the stress .sigma.6. These compressive stresses are indicated by minus sign in FIG. 26E. The tensile stresses .sigma.3 and .sigma.4 are respectively equal to the absolute values of the compressive stresses .sigma.6 and .sigma.5. In this case, no stress is applied to the resistance elements RY1 to RY4. Though the above analysis is performed with respect to the acceleration having the X-axis direction thereof, the similar results of stress analysis are obtained with respect to acceleration having the Y-axis direction thereof. Consequently, the absolute value of stress received by the resistance element adjacent to a frame 10D of the detector is equal to a half of that of stress received the resistance element adjacent to the center portion 31D of the resilient member 30D. This fact indicates that the arrangement of the resistance elements of the prior art results in a low sensitivity of the acceleration detector when acceleration is applied to the detector in the X- or Y-axis direction. The above stress analysis is performed with respect to the beam-type acceleration detector 1D because a stress analysis of the diaphragm-type acceleration detector 1C is very complex. However, there is the same problem as to the beam-type acceleration detector with respect to the diaphragm-type acceleration detector.